a) Field of the Invention
The invention is directed to a device for showing a first image in a second image which is visible through a transparent pane or sheet at which light proceeding from the first image is reflected and which is so arranged that the first image and the second image are detectable by the observer at the same viewing angle.
b) Description of the Prior Art
Devices of this kind can be used in aircraft and automobiles, for example. For this purpose, the second image represents, for example, the surroundings that are visible through the windshield. The first image, e.g., for flight operation, can be symbols which are displayed in the visual range or viewing area of the pilot via the windshield as virtual images serving as orientation aids for take-off and landing. The use of such devices also provides substantial advantages for other vehicles such as automobiles. For instance, by means of the first image, a symbol can be made visible representing the safe distance by which the driver of the vehicle can gage whether he is too close to a vehicle traveling in front of him. In so doing, it is possible to couple the apparent distance of this symbol with speed so that the vehicle operator can monitor whether or not an adequate safety distance is being maintained between him and the vehicle traveling in front of him.
Further, essential information from the display instruments can also be faded into the viewing area of the driver so that the driver has substantially better control over the vehicle than if he had to constantly glance back and forth between the environment and the dashboard, so that operating safety and driving safety are substantially improved.
There are a great many suggested systems for such devices which are also known as "Head-Up Displays" (HUD). From these suggested systems there arises the important fact that, in order to reduce fatigue, the information of the first image, as virtual image, is formed so that no accommodation of the eye is required while the eye is adjusted to distant vision. In order to form an image as a virtual image at a great distance, the object representing the first image, e.g., a liquid crystal display (LCD), is brought between the focal point and the first lens vertex of an imaging system.
Over the past twenty years, a large number of protective rights have been claimed worldwide for HUDs for motor vehicles. In Europe, however, these systems are still in the developmental and testing stage. In this connection, a distinction is drawn between systems with holographic optics and those without holographic optics. An overview of the former systems can be found, e.g., in the article "Holographic Applications in Avionic HUDs", Woodcock and Kirkham, Military Technology Miltech (1985), page 6.
In systems without holographic presentation such as those known from EP-A-0 202 460 or U.S. Pat. No. 4,740,780, imaging optics are either dispensed with entirely or conventional glass optics, e.g., according to GB 2 203 855 A, are used to generate a virtual image by means of an image generator. In the most elementary systems, the information which is displayed as a first image on a hidden LCD is simply projected on the windshield of a vehicle. An advantage in these HUDs consists in that the driver need not change his viewing direction to detect the displayed information; however, in order to read, the driver must adjust his eyes to the short distance to the windshield. The changing accommodation of the eye to distance vision for viewing the surroundings and to the windshield leads to fatigue and accordingly to lowered response time especially in older drivers.
In systems with virtual image display several meters in front of the windshield, the relaxed eye can read the information at any time; unnecessary fatigue is thus avoided. However, due to the reflection of light on the aspherically curved windshield, especially with virtual image display, imaging aberrations occur which must be corrected by specially computed optics. As can be gathered from DE 26 33 067C2, the production and computation of these compensatory optics is very involved and difficult with respect to manufacturing tolerances. Because of the large space requirement of conventional optical systems, integration problems also occur.
The following questions in particular must be answered as regards the use of HUDs in vehicles:
How great is the space requirement for a system and can this system be integrated in a vehicle? PA1 Can the vehicle supply sufficient output to power the light source and image generator? PA1 How high are production costs?
These questions must be taken into account already in the design stage. Two HUDs of very different design will now be described:
The first system, developed by Holtronic GmbH Ottersberg in association with BMW, Munich, dispenses with a separate combiner, as it is called, in the windshield to combine the first and second images so as not to impair the driver's clear view of the outside environment. This system is described, e.g., in DE 37 12 663A1. The imaging element can be a holographic optical element (HOE) in transmission operation (transmission HOE or T-HOE) which is integrated in the dashboard or instrument panel of the motor vehicle.
The essential structural component parts in this solution are a light source, an image generator and the T-HOE which carries out several functions. It directs the light beam in the desired direction on the windshield, generates an enlarged virtual image of the object at a distance of several meters in front of the windshield, and compensates for errors occurring as a result of the reflection of light on the windshield.
In this connection, there exist various possibilities for optical presentation of information. For virtual display of instruments or symbols intended to alert the driver of a defect in the electronics, for example, a stationary imaging plane is sufficient. With other information displays, e.g., the display of a position-dependent object, as in the safety distance indicator mentioned above, a variable distance display is useful. This can be realized in different ways.
With the use of holographic methods, stationary symbols can be displayed as three-dimensional images at various distances by means of a multihologram, as it is called. Such a hologram comprises a plurality of individual holograms which reconstruct the same object at different distances. However, an adjusting device is required for specifically illuminating each individual hologram. Imaging with a stereoscopic beam path, on the other hand, is a more elegant solution, since the image generation can be changed optionally. This method has been realized with the HUD mentioned above.
The spatial or three-dimensional image impression is created in the stereoscopic process by the binocular parallax when the left eye and right eye are presented with the respective slightly different aspect of an object. The spatial shift is achieved by generating two images of the display object from the respective visual angle of the eyes while taking into account convergence and the magnitude of the object. In so doing, the imaging plane (focal plane) remains stationary. Thus, the HUD is formed of two optical channels, each with an image generator. The information of each individual image enters the respective eye. The brain then allows the two partial images to melt together in an overall picture.
In principle, because of the possibility of variable display of information, there is demand for displays in which the symbols shown in the first image can be generated electronically. Picture tubes and LCDs are suitable for this purpose. However, picture tubes are not practical in automobiles due to high price and because of the voltage supply unit for generating high voltage. LCDs, on the other hand, are compact and relatively easy to drive or control and their voltage supply also does not pose a problem. Disadvantages are insufficient brightness, low contrast which is dependent on the wavelength and polarization of the scatter light, sensitivity to temperature, and low resolution capability.
The latter represents a major problem for use in a motor vehicle. At present, suitable LCDs have a pixel size of more than 0.3 mm. At a magnification factor of 50, the driver sees the pixel at a size of 15 mm. The spacing between the pixels is around 10% greater than the pixel itself. Therefore, the unwanted image of the pixel matrix is still clearly visible from a distance of 10 to 15 m. Only relatively large jumps in distance can be exactly displayed with this pixel size because, while the size of the symbols can be correctly generated, the jumps in distance of the symbols to be generated (lateral disparity) are particularly large because of the pixel size, especially at great distances. A compromise must be made, wherein the image has the correct magnitude at any apparent image distance, but the lateral disparity can be correctly adjusted only for specified distances because of the low resolution. As regards the use of LCDs for this system in the automotive field, it may be concluded that resolution power, contrast and temperature stability would have to be improved.
The T-HOEs mentioned above have the characteristic of dividing white light into its spectral components. Imaging via a T-HOE with irradiation by white light would lead to blurring of the image due to such chromatic aberrations. These chromatic aberrations can be compensated in a wavelength region of around 100 nm by combining a plurality of HOEs; but if only one HOE is used, this results in the requirement for a narrow-band light source with an unrealistic requirement of .DELTA..lambda.&lt;10 nm. Further, the light source would have to possess the highest possible brightness or luminance to allow the observer still to detect the display against a bright real background. The required luminance of the light source can be determined from the luminance at the position of the observer and from the efficiency of the optical system.
Another factor determining the suitability of a light source is its size and optical output. Light sources consuming 100 W or more are impractical not only because of the high output, but also because of high heat generation. Moreover, a light source may not have a long delay time (t &lt;10 s) between the controlling or triggering phase and operating phase so that warning symbols can be displayed in the most direct manner possible.
The article "Windshield with Holographic Mirror for Head-Up Displays", W. Windeln and M. A. Beeck, Automobiltechnische Zeitschrift 91 (1989), pages 538-342, mentions another system which was developed by Volkswagen AG, Wolfsburg, in association with Vereinigten Glaswerke GmbH, Aachen. This system uses a holographic combiner which is integrated in the windshield. The desired information is generated on an LCD and appears with the help of an imaging lens with a stationary display plane. A high degree of efficiency is achieved in the overall system through the use of a combiner. As was described above, the requirements imposed on a combiner (high transmission, high efficiency, high reflection factor, low spectral bandwidth and good imaging quality) can only be met by a holographic combiner. At present, the only suitable holographic storage medium is dichromate gelatin (DCG) which has also been used in aircraft for many years. Photopolymers represent another material which might also be useful for this purpose. The photopolymers offered by Polaroid, Offenbach (with wet development) and Du Pont, Wilmington, U.S.A., are particularly notable. The latter do not require an involved wet development process with toxic chemicals. After a simple diffuse post-exposure and subsequent heat treatment, the hologram is fully developed and fixed. However, photopolymers are still in the developmental phase, i.e., are not yet commercially available.
A majority of development efforts is focussed on the industrial manufacture of the combiner and its integration into the windshield.
In principle, the distortion of the virtual light image can also be corrected by the combiner. A narrow-band green phosphor serves as a light source for the display and, in combination with the combiner, suppresses chromatic imaging errors.
The solutions known from the prior art have the following fundamental drawbacks: the required luminance in the viewing field of the vehicle operator is not achieved by LCD displays or rear-illumination displays or even cathode-ray tubes because the arrangement must also be operable under glaring sunlight. Further, the holograms used to mirror the display element in particular possess the very unpleasant quality that they appear when illuminated with broadband light in the visible spectrum and in rainbow colors.
For the purpose of increasing the light intensity, it is proposed in DE 38 22 222 A1 to arrange polarizing filters on the inside of the windshield and to reflect the first image directly via an optical system as a virtual image on the windshield. In this way, glaring sunlight, for example, can be damped so that the first image is more visible. Further, these polarizing filters have the advantage that double images and distracting reflections on the windshield are prevented or at least reduced. However, experience has shown that this solution also does not provide sufficiently high light intensity for the first image with simultaneous visibility of the bright second image.